A brushless motor generally includes a control system that controls the excitation of phase windings of the motor. The control system may employ a current control scheme in which a winding is excited until current in the winding exceeds a threshold. On exceeding the threshold, the control system freewheels the winding so as to allow current in the winding to decay. Freewheeling may continue until current in the winding decays to a lower threshold, at which point the winding is again excited. Alternatively, the control system may freewheel the winding for a fixed period of time, after which the winding is again excited.
As the motor speed increases, the electrical half-cycle period of the motor decreases. Consequently, when the winding is freewheeled for a fixed period of time, the period over which current is driven into the winding decreases. This drawback may be resolved by freewheeling the winding for a fixed electrical angle rather than a fixed period of time. However, the control system must then calculate the corresponding freewheel time on-the-fly, which requires a faster and more expensive controller.